1. Field of the Invention
The present invention relates to a method of manufacturing a mono-crystalline silicon ball useful as a substrate for semiconductor devices.
2. Description of the Prior Art
A utility of silicon wafers as representative semiconductor material becomes stronger and stronger in correspondence to development and spread of semiconductor devices in various industrial fields. Since a silicon wafer bigger in diameter is profitable for fabrication of semiconductor devices with higher production efficiency, silicon wafers of 300 mm in diameter has been used. Production of silicon wafers of 400 mm in diameter is also researched. However, the tendency to enlargement of wafers in diameter would be doubtful, accounting the economical point of view that a plant designed for production of such big wafers is very expensive.
There is a concept directed to use a mono-crystalline silicon ball of about 1 mm in diameter, on the contrary to enlargement of silicon wafers. In fact, an integrated circuit designed on a surface of such a mono-crystalline silicon ball is researched and examined on applicability as a low-cost next-generation IC to a micro-machine or the like. A mono-crystalline silicon ball has been produced so far by a high-frequency plasma method, a rotary disk method, a gas-atomizing method, a water-atomizing method, an argon-arc rotary electrode method, a plasma-arc rotary electrode method, etc. For instance, JP 11-12091 A1 discloses a mono-crystalline silicon ball manufacturing method, wherein spheroidal poly-crystalline silicon covered with oxide film is partially melted with heat and then re-crystallized to a spheroidal shape while shifting the molten part.
It is a big problem how to prepare mono-crystalline silicon in a spheroidal shape at a low cost. A manufacturing method, which fulfills industrial productivity, has not been established yet. There are many unknown matters on progress of crystallization together with reproducibility, when a mono-crystalline silicon ball is produced with use of a drop tube or by an atomizing method, so that many problems necessary for improvement of productivity are still unsettled. The present invention is accomplished to overcome the above-mentioned problems.
The present invention aims at production of a mono-crystalline silicon ball directly from an under-cooled droplet by properly controlling under-cooling of the droplet. An effect of the under-cooling on crystallizing the droplet to a spheroidal shape is discovered by the inventors from studies on re-crystallization of an under-cooled droplet.
The present invention proposes a new method, whereby a mono-crystalline silicon ball is manufactured from under-cooled silicon melt suspending a crystalline nucleus therein under the condition that a critical under-cooling xcex94Tcr is determined in correspondence with a diameter d of a silicon droplet so as to satisfy relationships of (d=5 mm, xcex94Tcr=100K), (d=3 mm, xcex94Tcr=120K), and (d=1 mm, xcex94Tcr=150K) between the critical under-cooling xcex94Tcr and the diameter d.